1. J Solid State Electrochem (2009) 13:1561–1566
DOI 10.1007/s10008-008-0730-0
ORIGINAL PAPER
Pt–Ru nanoparticles supported PAMAM dendrimer
functionalized carbon nanofiber composite catalysts
and their application to methanol oxidation
T. Maiyalagan
Received: 22 August 2008 / Revised: 6 October 2008 / Accepted: 4 November 2008 / Published online: 20 November 2008
# Springer-Verlag 2008
Abstract Polyamidoamine (PAMAM) dendrimers has sources due to their high conversion efficiencies, low
been anchored on functionalized carbon nanofibers (CNF) pollution, light weight, and high power density. Methanol
and supported Pt–Ru nanoparticles have been prepared with offers the advantage of easy storage and transportation
NaBH4 as a reducing agent. The samples were character- when compared to hydrogen oxygen fuel cell, its energy
ized by X-ray diffraction, scanning electron microscopy, density (~2,000 Wh/kg) and operating cell voltage (0.4 V)
and transmission electron microscopy (TEM) analysis. It are lower than the theoretical energy density (~6,000 Wh/kg)
was shown that Pt–Ru particles with small average size and the thermodynamic potential (~1.2 V) [1, 2]. However,
(2.6 nm) were uniformly dispersed on PAMAM/CNF the fuel cells could not reach the stage of commercialization
composite support and displayed the characteristic diffraction due to the high cost, which is mainly associated with the
peaks of Pt face-centered cubic structure. The electrocatalytic noble metal-loaded electrodes and the membrane. In order
activities of the prepared-composites (20% Pt–Ru/PAMAM- to reduce the amount of Pt loading on the electrodes, there
CNF) were examined by using cyclic voltammetry for have been considerable efforts to increase the dispersion of
oxidation of methanol. The electrocatalytic activity of the the metal on the support. The catalyst is very often found to
CNF-based composite (20% Pt–Ru/PAMAM-CNF) electrode disperse on a conventional carbon support, and the support
for methanol oxidation showed better performance than that material influences the catalytic activity through metal
of commercially available Johnson Mathey 20% Pt–Ru/C support interaction [3–5].
catalyst. The results imply that CNF-based PAMAM com- New novel carbon support materials such as graphite
posite electrodes are excellent potential candidates for nanofibers (GNFs) [6, 7], carbon nanotubes (CNT) [8–12],
application in direct methanol fuel cells. carbon nanohorns [13], and carbon nanocoils [14] provide
alternate candidates of carbon support for fuel cell
Keywords Methanol oxidation . DMFC . Dendrimers . applications. Bessel et al. [6] and Steigerwalt et al. [7]
Nanostructured materials . Electro-catalyst used GNFs as supports for Pt and Pt–Ru alloy electro-
catalysts. They have observed better activity for methanol
oxidation. The high electronic conductivity of GNF and the
Introduction specific crystallographic orientation of the metal particles
resulting from well-ordered GNF support are believed to be
Fuel cells operate with the electrochemical oxidation of the factors for the observed enhanced electro-catalytic
hydrogen or methanol, as fuels at the anode and the activity. In heterogeneous catalysis, one of the important
reduction of oxygen at the cathode are attractive power tasks is the determination of the number of active sites in
the catalyst. For a given catalyst, the number of active sites
present is responsible for the observed catalytic activity. A
T. Maiyalagan (*) considerable amount of research has been devoted toward
Department of Chemistry, School of Science and Humanities,
understanding the number of active sites and the role
VIT University,
Vellore 632014, India played by the carrier of the supported catalysts. The most
e-mail: maiyalagan@gmail.com efficient utilization of any supported catalyst depends on

2. 1562 J Solid State Electrochem (2009) 13:1561–1566
the percentage of exposed or the dispersion of the active scanning electron microscopy (SEM), transmission electron
component on the surface of the carrier material. Among microscopy (TEM), and cyclic voltammetry. The electro-
the various factors that influence the dispersion of an active chemical properties of the composite electrode (Pt–Ru–
component, the nature of the support and the extent of the PAMAM/CNF) were compared to those of the commercial
active component loading are of considerable importance. electrode (Pt–Ru/C), using cyclic voltammetry. The 20% Pt–
Without surface modification, most of the carbon nano- Ru–PAMAM/CNF exhibited excellent catalytic activity and
materials lack sufficient binding sites for anchoring precursor stability when compared to the 20 wt.% Pt–Ru/C.
metal ions or metal nanoparticles, which usually lead to poor
dispersion and the aggregation of metal nanoparticles,
especially at high loading conditions. To introduce more Experimental
binding sites and surface anchoring groups, an acid oxidation
process was very frequently adopted to treat carbon nano- Materials
materials in a mixed acid aqueous solution, which introduces
surface bound polar hydroxyl and carboxylic acid groups for All the chemicals used are of analytical grade. CNFs (grade
subsequent anchoring and reductive conversion of precursor PR 24 LHT) are commercially available from Pyrograf,
metal ions to metal nanoparticles [15]. USA. 1-(3-Dimethyl-aminopropyl)-3-ethyl-carbodiimide-
Generally, metal nano-particles have been synthesized for hydrochloride (EDC), N-hydroxysuccinimide (NHS), hexa-
the purpose of increasing the catalytic surface area in chloroplatinic acid, and ruthenium (III) chloride hydrate are
comparison to bulk metal and have utilized strategies such procured from Sigma-Aldrich and used as received. Vulcan
as stabilization in a soluble polymer matrix or encapsulation XC-72 carbon black is purchased from Cabot. Methanol
in dendrimers to protect them from aggregation. Dendrimers and sulfuric acid are obtained from Fischer Chemicals.
are good candidates for preparing metal nanoparticles because Nafion 5 wt.% solution is obtained from Dupont and is
they can act as structurally and well-defined templates and used as received. Fourth generation amine-terminated
robust stabilizers. Polyamidoamine (PAMAM) dendrimers, in polyamidoamine PAMAM dendrimers (G4-NH2) with the
particular, have been used as nanoreactors with effective highest available purity (10 wt.% in methanol) are obtained
nanoparticle stabilization. In addition, encapsulated nano- from Aldrich and used without further purification.
particles surfaces are accessible to substrates so that catalytic
reactions can be carried out [16–18]. Functionalization of CNF
PAMAM dendrimer encapsulated Pt nanoparticles and PtPd
bimetallic nanoaparticles are electrocatalytically active for The CNF is treated with mixed acid aqueous solution of
oxygen reduction [19–21]. Also dendrimer-encapsulated plat- HNO3 and H2SO4 in 1:3 ratios under magnetic stirrer for
inum nanoparticles supported on carbon fiber and nitrogen- 3 h. It is then washed with water and evaporated to dryness.
doped CNT as electrodes for oxygen reduction [22, 23].
In addition, platinum–ruthenium alloys are the best Preparation of CNF-PAMAM
catalysts for methanol oxidation direct methanol fuel cells
[24, 25]. PAMAM dendrimers were modified with a Ni- Fifty milliliters of 0.1 mg/ml carboxylated CNF containing
cyclam as novel electrocatalytic material for the electro- 100 mg of EDC and 100 mg of NHS is slowly added into a
chemical oxidation of methanol [26]. Kim et al. [27] have methanol solution of 20% G4 PAMAM under high speed
reported the preparation of platinum–dendrimer hybrid agitating.
nanowires using alumina templates, which exhibited and
enhanced electro-catalytic activity toward methanol oxidation. Preparation of Pt–Ru–PAMAM/CNF catalysts:
PAMAM dendrimer (PAMAM 4.5)-encapsulated Pt nano-
particles chemically linked to gold substrates through a 6.15 mM H2PtCl6 and 11.8mM RuCl3 and dendrimer
cystamine monolayer exhibited electrocatalytic activity toward encapsulated CNF are mixed by keeping the solution under
the oxidation of methanol [28].The terminal functional groups magnetic stirrer for 3 h. Five milliliters of 0.1 M NaBH4 is
of the dendrimers stabilizes metal nanoparticles without added to the solution and kept under evaporation. A
aggregation. In this paper, we make use of the fourth- schematic of the detailed procedure for the electrocatalyst
generation amine-terminated PAMAM dendrimers (G4-NH2) preparation is shown in Fig. 1.
to anchor on the functionalized carbon nanofiber (CNF) as a
substrate and then encapsulate Pt–Ru nanoparticles on Characterization
dendrimers for the better dispersion of the electrode, which
exhibited very good catalytic activity. These materials are The phases and lattice parameters of the catalyst are
characterized and studied, using X-ray diffraction (XRD), characterized by XRD patterns employing Shimadzu XD-

3. J Solid State Electrochem (2009) 13:1561–1566 1563
Fig. 1 Schematic diagram
illustrating synthesis of Pt–Ru–
PAMAM/CNF
D1 diffractometer using Cu Kα radiation (λ=1.5418 Å) centered cubic structure can be observed. X-ray scattering
operating at 40 kV and 48 mA. XRD samples are obtained from the CNF support is evidenced by the peak at 26° in
by depositing composite supported nanoparticles on a glass 2θ. The diffraction peaks of the catalyst (Pt–Ru/C) are
slide and drying the latter in a vacuum overnight. The observed to be sharp with a high intensity indicating high
scanning electron micrographs are obtained using JEOL crystallinity (Fig. 2a). On the contrary, very broad peaks
JSM-840 model, working at 15 keV. For TEM studies, the with weak intensity are observed for PAMAM containing
composite dispersed in ethanol are placed on the copper composites (Pt–Ru/PAMAM-CNF), indicating that they are
grid, and the images are obtained using JEOL JEM-3010 not fully crystalline in nature (Fig. 2 b) as observed for the
model, operating at 300 keV. commercial Pt–Ru/C (JM) (Fig. 2a). Compared with Pt–
Ru/C, the peak intensities of Pt–RuPt–PAMAM/CNF are
Electrochemical measurements lower and the full width at half-maximum (FWHM) for the
peaks are bigger. Bigger FWHM indicates a smaller
All electrochemical measurements are performed using a BAS average size of metal nanoparticles on PAMAM/CNF
Epsilon potentiostat. A three-electrode cell is used, which composite. No evidence of peaks related to Ru was found
consisted of the glassy carbon (0.07 cm2) as working electrode in these catalysts. The absence of diffraction peaks typical
and Pt foil and Ag/AgCl (saturated by KCl solution) for Ru can be due to a number of reasons such as Ru not
electrodes as counter and reference electrodes, respectively, being dissolved in the Pt lattice, that is, forming a PtRu
are used. All the electrochemical experiments are carried out alloy and/or the Ru being present in the amorphous form, as
at room temperature in 0.5 M H2SO4 electrolyte. The further discussed below [29, 30]. The position of the Pt
electrolyte solution is purged with high pure nitrogen for (111) peak for the larger (>1.2 nm) sized catalysts is shifted
30 min prior to a series of voltammetric experiments.
Preparation of the working electrode
C (002)
(a) 20% Pt-Ru-PAMAM/CNF
2
Glassy carbon (GC) (Bas Electrode, 0.07 cm ) is polished to a (b) 20% Pt-Ru/C
mirror finish with 0.05 μm alumina suspensions before each
Pt (111)
experiment and served as an underlying substrate of the (a)
Intensity (a.u)
working electrode. In order to prepare the composite electrode,
the catalysts are dispersed ultrasonically in water at a
concentration of 1 mg ml−1 and 20-μl aliquot is transferred
on to a polished GC substrate. After the evaporation of water, (b)
the resulting thin catalyst film is covered with 5 wt.% Nafion
solution. Then, the electrode is dried at 353 K and used as the
working electrode. A solution of 1 M CH3OH in 0.5 M
H2SO4 is used to study the methanol oxidation activity.
Result and discussion 10 20 30 40 50 60 70 80
2θ (degrees)
The crystal structures of the catalysts are examined by XRD
as shown in Fig. 2. Characteristic reflections of Pt face- Fig. 2 XRD spectra of a Pt–Ru/C, b Pt–Ru–PAMAM/CNF

4. 1564 J Solid State Electrochem (2009) 13:1561–1566
to higher 2θ positions than that for pure Pt that has a Ru–PAMAM/CNF. According to the EDX measurements
maxima at 39.7645° [31]. (Fig. 4), catalysts prepared in this work contained 20.1 wt.%
The scanning electron micrograph images of commercial of total metal with a Pt/Ru atomic ratio of 1:1.0~1.1, which
CNFs are shown in Fig. 3a,b. The diameter of the agrees well with the stoichiometric ratio of 1:1 used in the
nanofibers is measured using a Leica Qwin Image Analyzer starting mixture.
and is found to be in the range of 260–250 nm and the The TEM image of the prepared Pt–Ru–PAMAM/CNF
length of about 100 μm. Figure 3c shows the image of Pt– catalysts, shown in Fig. 5, reveals that well-dispersed,
spherical particles were anchored onto the external walls of
PAMAM/CNF composite support with an average size of
(a) 2.6 nm. There is no agglomeration of Pt–Ru nanoparticles
in the composite, and the Pt–Ru nanoparticles were found
to be uniformly dispersed. In comparison, a commercial
Johnson Matthey Pt–Ru catalyst (20 wt.% on Vulcan) had
an average particle size of 3 nm. The uniform dispersion of
metal nanoparticles on PAMAM/CNF composite is clearly
due to the PAMAM dendrimer, which offers large and
uniform distributed nitrogen-active sites for anchoring
metal ions and metal nanoparticles. In this regard,
PAMAM-functionalized CNF composite are far more
effective supports than the conventional-acid-oxidized
CNF. The conventional-acid oxidation functionalization
leads to the structural damage of CNF, which causes a
large extent of loss in electrical conductivity and thus
potentially reduces the electrocatalytic activity of the
(b) catalyst. The three-dimensional structure, smaller particle
size, and high dispersion of nanoparticles may result in
large valuable Pt surface area and good electrocatalytic
properties toward methanol oxidation.
The electrocatalytic activities for methanol oxidation of
Pt–Ru–PAMAM/CNF and commercial Pt–Ru/C electro-
catalysts are analyzed by cyclic voltammetry in an
electrolyte of 0.5 M H2SO4 and 1 M CH3OH at 50 mV/s.
Figure 6 shows the cyclic voltammograms (CVs) of the
synthesized Pt–Ru–PAMAM/CNF and commercial Pt–Ru/C
catalysts in an electrolyte solution of 0.5 M H2SO4 and 1 M
CH3OH. There are two irreversible current peak during the
(c)
Fig. 4 EDX spectrum of Pt–Ru–PAMAM/CNF (PtRu 20.1 wt. %; Pt/
Fig. 3 SEM images of a, b CNF and c Pt–Ru–PAMAM/CNF Ru=1:1.0~1.1)

5. J Solid State Electrochem (2009) 13:1561–1566 1565
[34–38]. A higher ratio indicates more effective removal of
the poisoning species on the catalyst surface. The If/Ib ratios
of Pt–Ru–PAMAM/CNF and Pt–Ru/C are 1.36 and 0.87,
respectively, showing better catalyst tolerance of PAMAM/
CNF composites. There is no much decrease in electro-
catalytic activity of the Pt–Ru–PAMAM/CNF composite
catalysts on subsequent cycles of methanol oxidation
compared to the Pt–Ru/C catalysts. This not only demon-
strates the reproducibility of the results but also the stability
of the nanoparticles on the PAMAM/CNF composite
catalysts.
Similarly, Fig. 7 shows the chronoamperometric studies
of the synthesized sample, which reveals the stability of the
Fig. 5 TEM image of 20% Pt–Ru–PAMAM/CNF catalyst Pt–Ru–PAMAM/CNF catalyst toward the methanol electro-
oxidation. The results are consistent with the view that
electro-oxidation of methanol that are typically attributed PAMAM is not only stabilizing the Pt–Ru nanoparticles but
on the forward scan peak at around 0.8 V to methanol is also enhancing the methanol oxidation currents under
electro-oxidation and on the backward peak at 0.6 V to the chronoamperometric conditions. Pt–Ru nanoparticles are
faradic oxidation reaction on the Pt of the residual well stabilized by amine-terminated PAMAM dendrimers.
intermediate species. Both CV curves reveal a similar The high electrocatalytic activity of the Pt–Ru–PAMAM/
shape and peak position, which is also in good agreement CNF composite electrodes prepared in this work is most
with previous reports for methanol CVs over supported Pt probably due to the very high dispersion of PtRu particles
catalysts. The Pt–Ru–PAMAM/CNF composite shows and the extensive presence of RuOxHy.
higher electrocatalytic activity of methanol oxidation
compared to Pt–Ru/C (JM) catalyst. The enhanced catalytic
activity of Pt–Ru/PAMAM–CNF composite catalyst is due Conclusions
to higher dispersion of Pt–Ru nanoparticles and may be
better oxidation of CO intermediates during methanol In summary, well-dispersed Pt–Ru nanoparticles have been
oxidation [32, 33]. synthesized on PAMAM–CNFs composite. The choice of
The ratio of the forward anodic peak current (If) to the the PAMAM G4-NH2 dendrimer template and terminal
reverse anodic peak current (Ib) can be used to describe the amine functional groups provides for uniform preparation
catalyst tolerance to accumulation of carbonaceous species of size monodisperse catalysts and facilitates the controlled
dispersion. The enhancements in activity and stability over
Pt–Ru/PAMAM–CNF catalyst have been solely attributed
to high dispersion Pt–Ru nanoparticles on the composite.
0.040
(a) 20% Pt-Ru-PAMAM/CNF
0.035 (b) 20% Pt-Ru/C
0.030
Current (A)
0.025
0.020
0.015
(a)
0.010
(b)
0.005
0.000
0 500 1000 1500 2000 2500 3000 3500
Time (s)
Fig. 6 Cyclic voltammogram of a Pt–Ru–PAMAM/CNF and b Pt–
Ru/C in electrolyte solution of 0.5 M H2SO4 with 1 M CH3OH at a Fig. 7 Chronoamperometry of a Pt–Ru–PAMAM/CNF, b Pt–Ru/C
sweep rate of 50 mV/S at room temperature polarized at +0.6 V in 0.5 M H2SO4/1 M CH3OH

6. 1566 J Solid State Electrochem (2009) 13:1561–1566
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